1. Field of the Invention
The present invention relates to a wide-angle lens system, an image sensing optical device and a digital apparatus. For example, the present invention relates to: a wide-angle lens system that is suitable for a lens-interchangeable digital camera which takes in the picture of a subject with an image sensing element, that is compact and that has a large aperture; an image sensing optical device that outputs, as an electrical signal, the picture of the subject taken in with the wide-angle lens system and the image sensing element; and a digital apparatus having an image input function, such as a digital camera that incorporates such an image sensing optical device.
2. Description of Related Art
In a conventional large-aperture wide-angle lens system used as an interchangeable lens for a single-lens reflex camera, for example, an overall feed-out system as disclosed in patent document 1 and a rear focus system as disclosed in patent document 2 are mainly used as a focusing system. In recent years, users have accepted the compactness of mirror-less lens-interchangeable cameras formed by omitting a flip-up mirror from a single-lens reflex camera, and thus their market has extended. In some of the mirror-less lens-interchangeable cameras, phase-difference AF (autofocus) that is mainly used in a conventional single-lens reflex camera cannot be used. In such cameras, so-called contrast AF is used in which focusing is performed by scanning a focusing lens group to find a place where the contrast is the highest.    Patent document 1: JP-A-2012-037640    Patent document 2: JP-A-2010-039340
A problem occurring when the contrast AF is used is the weight of the focusing lens group. Since in the phase-difference AF, it is possible to calculate, with information from an AF sensor, the amount of movement of the focusing lens group necessary to achieve focusing, it is possible to move the focusing lens group according to the amount. On the other hand, since in the contrast AF, information obtained from an AF sensor is only an instantaneous contrast value, a focusing operation is performed by finding a place where the contrast is the highest while the focusing lens group is being moved and instantaneous variations in contrast are being read. Hence, when the amount of movement of the focusing lens group until the focusing is achieved is compared between the contrast AF and the phase-difference AF, the amount is significantly larger in the former case. It is obvious from what has been described above that, in an optical system corresponding to the contrast AF, the reduction in the weight of the focusing lens group is an important point. In this point, the focusing lens groups disclosed in patent documents 1 and 2 insufficiently reduce the weight thereof, and thus it is required to produce a new optical solution.